(A) Keratins form obligate heteropolymers (one type I and one type II keratin) and share a common structure that consists of a central coiled-coil α-helical rod domain that is flanked by non–α-helical head and tail domains. The head and tail domains are unique in that they include most of the sites for posttranslational modifications [phosphorylation (P) and glycosylation (G)]. The rod domain is subdivided into 3 subdomains (coils 1A, 1B, and 2) and by 2 linkers (L1 and L12). The L12 region includes a highly conserved caspase-digestion site (VEVD in K18) among type I keratins and other IFs, while only the K18 tail domain has a caspase cut site (DALD). The K18 VEVD and DALD (the M30 epitope) are recognized by epitope-specific antibodies (40, 41, 54). (B) Liver, pancreas, and intestine phenotypes caused by keratin mutations, as determined from findings in SEK-related animal models or in human patient studies. +, positive phenotype; –, absent phenotype; +/–, borderline phenotype; ?, untested and therefore unknown. Borderline phenotypes were assigned in the case of K8 mouse pancreas because pancreatitis develops after K8 overexpression, but at high gene copy numbers, and in the case of human intestine because K8/K18 variants are unlikely to be involved in predisposition to IBD. Also shown are some of the organs that we hypothesize may ultimately prove to be involved in additional SEK variant–associated human diseases, with a few potential candidates listed.

Shown are the domain organization of the protein backbone of K8 and K18 and the general distribution of different point mutations and deletions. K8 Y54H, G62C, R341H, and G434S variants are found in patients with liver disease, but show a strong ethnic distribution. The first described SEK variant was K18 H128L (74). In white individuals, the most common K8 variant is R341H, while the most common amino acid–altering variant in black individuals is G434S. The 2 human mutations, K8 G62C and G434S, inhibit the phosphorylation of the adjacent S74 and S432, respectively. Notably, transgenic mice that express K8 G62C phenocopy the marked predisposition to liver injury observed in mice that express the phosphomutant K8 S74A. Asterisks at the beginning and end of the rod domain highlight the helix initiation and helix termination motifs that include mutation hot spots for epidermal keratins, and for IFs in general, but not for the SEKs. The assignment of the specific location of the keratin subdomains was obtained from the Human Intermediate Filament Database (http://www.interfil.org/proteinsTypeInII.php) and includes the start methionine residue. Most individuals with keratin mutations that are at increased risk of liver disease are heterozygous for the mutations, with few patients harboring compound heterozygous mutations. K19 variants are not included in this schematic since they are newly identified (91) and have not been characterized in the detail that K8 and K18 variants have.

(A) Keratin filament reorganization into deposits, granules, or aggregates involves several physiologic (e.g., mitosis and apoptosis) and pathologic (e.g., aggregates that include or exclude K8 and K18 found in the context of several liver disorders) conditions that result in MDB or non-MDB structures. Ub, ubiquitin. (B–E) MDBs (arrows) are identified by pathologists, using H&E staining, as eosinophilic structures (B). Alternatively, MDBs can be visualized by immunofluorescence (C) or by immunohistochemical staining using K8- and K18-specific antibodies (C, green signal, and D), ubiquitin (C, red signal), and p62-specific antibodies (E). The immunological detection methods offer greater sensitivity and specificity than histological MDB staining; however, the latter is currently used clinically and is broadly accepted. Note the similar morphology of MDBs that are identified in mice fed DDC and seen in humans with alcoholic steatohepatitis (ASH).

The various SEK functions, which contribute to the major role of SEKs as cytoprotectors, are interrelated and include: providing mechanical integrity; modulating protein localization, targeting, and/or synthesis via association with other proteins (e.g., hsp70, 14-3-3 proteins, Fas receptor, protein kinases [PKs], and phosphatases such as protein phosphatase–2A [PP2A]); contributing to the positioning and function of organelles such as mitochondria and Golgi; serving as a phosphate “sponge” under stress (e.g., via phosphorylation of K8 S74); and protecting the cells from undergoing apoptosis.